Asymmetrical charge movement in slow- and fast-twitch mammalian muscle fibres in normal and paraplegic rats.

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Asymmetrical charge movements (Q) were recorded from the voltage-clamped ends of muscle fibres in extensor digitorum longus (e.d.l.) and soleus muscles from rats. Tetracaine (2 mM) was added to solutions to prevent contraction. In both muscles the relationship between Q and membrane potential (V) was S-shaped and could be described by the Boltzmann-type equation Q = Qm/(1 + exp[-(v - V)/k]) where Qm was the maximum charge, V the membrane potential at which Q = Qm/2, and k a 'slope factor'. On average, Qm was 5-6 times greater in e.d..l. than in soleus fibres and charge movement occurred at more negative potentials in soleus than in e.d.l. fibres, V being -36.7 mV in the former and -19.0 mV in the latter, a difference of about 18 mV. The threshold for contraction, determined using a two-electrode voltage clamp, was more negative in soleus than in e.d.l. fibres. For 500 ms depolarizations, the difference was 12 mV. The relationship between tension and membrane potential during potassium contractures was S-shaped and, when fitted by the Boltzmann-type equation, gave V values of -25 mV for soleus and -14 mV for e.d.l. fibres. In paraplegic rats, the threshold for contraction in soleus fibres shifted about 12 mV to more positive potentials, but there was no change in e.d.l. fibres so that there was no significant difference between the two muscles. In paraplegic rats the relationship between tension and membrane potential during potassium contractures also shifted to more positive potentials in soleus fibres, whereas there was no change in e.d.l. fibres. These changes in the voltage sensitivity of contractile activation in soleus fibres from paraplegic rats were associated with a parallel shift in the voltage sensitivity of charge movement so that the average V shifted from -36.7 mV in normal rats to a value of -14.2 mV in paraplegic rats. There was also a four-fold increase in Qm in soleus fibres from paraplegic rats. The difference between the voltage sensitivity of contractile activation and charge movement in e.d.l. and soleus fibres in normal rats supports the hypothesis that the two are closely related: even stronger support comes from the observation of the parallel shift in the voltage sensitivity of contractile activation and charge movement in soleus fibres in paraplegic rats.

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